In my aforesaid parent application, I describe production of tantalum and niobium-based materials useful as electrolytic capacitor anodes. As described in my aforesaid parent application, tantalum-based electrolytic capacitors have found increasing use in microelectronics. The combination of small package size, insensitivity to operating temperature, and excellent reliability have made them the choice over ceramic multilayer and aluminum foil-based capacitors for many applications. As the state of the art in microelectronics continues to progress, demand has grown for more efficient tantalum electrolytic capacitors.
An electrolytic capacitor consists of three basic components: an anode, a cathode, and an electrolyte. Heretofore, electrolytic tantalum anodes have primarily been fabricated using fine particle tantalum powder. The powder is pressed into a green compact (20 to 50 percent dense) and is sintered under vacuum at a temperature of 1500°-2000° C. for 15-30 minutes to form a porous, mechanically robust body in which the tantalum is electrically continuous. The sintering process is, in some cases, relied upon to attach a lead wire to the compact. In these cases, the lead is inserted into the green compact prior to sintering. If the lead is not attached in this manner, it usually will be welded into place immediately following sintering of the compact. An important ancillary benefit of the sintering operation is purification of the tantalum particle surfaces; impurities, such as oxygen, are driven off.
After sintering, the compact is anodized to form the dielectric tantalum pentoxide (Ta2O5) on the exposed surfaces. The porous regions of the anodized compact are then infiltrated with a conductive electrolyte. The electrolyte may be of the “solid” or “wet” type. Depending upon the application, a wet electrolytic capacitor may offer advantages over a solid electrolytic capacitor. Wet electrolytic capacitors tend to be larger than solid electrolytic capacitors and can offer higher capacitance as a result. This is desirable, since what is needed in many modern applications are capacitors with high energy densities, which are driven in part by anode capacitance and in part by cathode capacitance. This is due to the relationship between these two capacitances and the overall capacitance in a wet cell electrolytic capacitor. The overall capacitor consists of the anode and the cathode connected in series as two capacitors, and resulting in the relation:Ccap=1/((1/Canod)+(1/Ccath))
Where Ccap is the overall capacitance, Canod is the capacitance of the anode, and Ccath is the capacitance of the cathode (see, e.g., D. M. Edson and J. S. Bates, “Electrical Properties of a Novel High CV Wet Tantalum Capacitor System”, CARTS USA 2009 Proceedings, The 29th Annual Passive Components Symposium & Exhibition, pp. 415-425, Electronic Components Association, Arlington, Va., 2009). It is clear from this equation that as the value of Ccath increases, 1/Ccath tends to small values (zero in the limit where Ccath tends to infinity), and Ccap is dominated by Canod. For this reason, wet electrolytic capacitor manufacturers desire cathodes with the highest capacitance possible.
The conventional path to cathode fabrication for wet electrolytic capacitors is essentially the same as that for anode fabrication in that it involves pressing tantalum metal powders, sintering them into a porous structure, and then anodizing the structure to form tantalum pentoxide dielectric on the exposed surfaces. In principle, fabrication of very high capacitance cathodes is not difficult, since this may be achieved by simply making cathodes that have much greater surface area than do the anodes. In practice, however, there are packaging restrictions on the capacitor. A typical wet capacitor configuration will involve a central anode surrounded by a tantalum container, or “can”. The tantalum powders of the cathode are sintered so as to form a sleeve that fits inside the can, and the sleeve is then sintered to the can. A thin oxide layer is formed on this structure and the cathode is complete. An anode/cathode assembly of this type is specified in U.S. military document Mil-PRF-39006.
The capacitance of the cathode in this type of capacitor relies primarily upon the surface area available in the tantalum sleeve. Higher surface area translates into higher capacitance. If the sleeve is comprised of powders, as is usually the case, the powders must be very fine and/or aspected into flakes by such means as ball milling. As discussed below, the use of such powders and flakes can have serious drawbacks.
In my aforesaid parent application, I describe fabrication of improved electrolytic capacitor anode material by establishing multiple valve metal components, preferably tantalum or niobium, in a billet of a differing ductile metal, working the billet through a series of reduction steps to form the valve metal components into elongated elements, cutting the resulting elongated elements into sections, leaching the secondary ductile metal from the sections, washing and mixing the resulting valve metal filaments leached from the sections, and forming the filaments into a sheet useful for forming anodes. In accordance with the present invention, I have now found that tantalum-based electrolytic anode bodies formed as above advantageously may also be used as cathode bodies in high CV wet tantalum capacitors.